SALT LAKE CITY — A plan to expand operations at Utah's largest mining project is drawing fire from environmental advocates, who are taking their protests to Rio Tinto's headquarters Thursday in London.

Also on Thursday, a coalition of health and environmental organizations has scheduled a news conference at the Salt Lake City Main Library to announce their opposition to the Cornerstone mine expansion.

Last August, Kennecott Utah Copper announced a proposal to extend the life of its Bingham Canyon Mine operations to 2028. The extension would be called the Cornerstone Project. If approved, the plan would allow Kennecott to maintain current production levels.

Kennecott is a subsidiary of global mining firm Rio Tinto — which is headquartered in the United Kingdom. In a show of solidarity, coalition members and supporters — including a member of Utah Moms for Clean Air — are also expected to demonstrate at Rio Tinto's annual shareholders meeting in London.

Currently, the mine generates nearly 25 percent of the refined copper produced in the U.S. In 2009, Kennecott produced 7 percent of all U.S. refined gold, 10 percent of refined silver and 23 percent of molybdenum — a metallic element used to toughen alloy steels and soften tungsten alloy.

In a statement regarding the Utah news conference, the coalition said it would "provide detailed information on why the project will add to the health risks of Salt Lake County's already-impaired airshed, drinking water supplies for west-side residents, the Great Salt Lake and the area's ability to attract new economic opportunities."

Critics of the proposed expansion claim Salt Lake County and Salt Lake City have "some of the worst air quality in the nation," and the expansion would only exacerbate the problem.

"Their claims that they are not going to emit more air pollution are not credible," said Dr. Brian Moench, Utah Physicians for a Healthy Environment. "It's clear that they will be emitting more pollution."

The American Lung Association routinely ranks Salt Lake City in the top 10 worst cities for acute spikes in air pollution, a news release states.

Kennecott Utah Copper's mining operation is the largest single stationary source of air pollution in Salt Lake County, the statement said. The expansion would increase county air pollution by 12 to 14 percent.

"We think it's only fair, ethical and appropriate for them to put a modest amount of that profit into generating their electricity from completely clean and renewable sources," Moench said. "That would eliminate a significant amount of their emissions … and that would be the way to start."

Kennecott spokeswoman Jana Kettering said that while the project would likely increase some air emissions, the overall impact would likely result in a net decrease in environmental impact.

"We have proposed a plan — that as a whole — will reduce emissions by 9 percent," she said. "We'll be burning natural gas (to produce electricity) instead of coal, which is equivalent to removing one in 10 cars off the road."

Kenncott said the Cornerstone project would entail "pushing back the south wall of the mine about 1,000 feet and deepening the mine about 300 feet to reach 700 million tons of ore."

This would require increasing the size of the pit's fleet of large-haul trucks over time, adding an in-pit crusher and another grinding line and supporting equipment at the Copperton Concentrator. In addition, the project would require Kennecott to update a number of existing air, water and land permits in order to allow the company to move and crush more rock and store more tailings to produce the same amount of copper it does now.

Opponents argue the expansion plan would worsen the air quality already impacting the Salt Lake area, while Kenncott claims it would make every effort to mitigate those health concerns.

"As a mining company, we're well aware that our activities disturb the land, create waste products and use energy resources," said Kelly Sanders, Kennecott Utah Copper president and chief executive officer. "We're committed to reducing these environmental impacts as much as possible."